<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <style>
input { width:30px }      
    </style>
  </head>
  <body>
<script>
  function calculate() {
    var a = document.getElementById('a')
    var op = document.getElementById('op')
    var b = document.getElementById('b')
    var result = document.getElementById('result')
    result.innerText = eval(a.value + op.value + b.value)
  }
</script>    
    <input id="a" type="text" value="3"/>
    <select id="op">
      <option value="+">+</option>
      <option value="-">-</option>
      <option value="*" selected>*</option>
      <option value="/">/</option>
    </select>
<!--    <input id="op" type="text" value="*"/>-->
    <input id="b" type="text" value="5"/>
     = <label id="result"></label><br/><br/>
    <button onclick="calculate()">計算</button>
  </body>
</html>